Platinum-containing materials, e.g., materials which contain at least 80 wt. % platinum, are widely used in the manufacture of glass and glass products because of their high melting temperatures, low levels of oxidation at elevated temperatures, resistance to corrosion by glass melts, and low levels of contamination of molten glass. Platinum-containing materials are also notoriously expensive. Accordingly, substantial reductions in capital costs can be achieved by even small reductions in the amount of platinum-containing materials used in a glass manufacturing facility.
Among the valuable characteristics of platinum-containing materials is their ability to generate heat when conducting electricity. As a result, molten glass flowing through, or held in, a platinum-containing vessel can be heated by passing electrical current between one or more locations along the length of the vessel's glass-contacting wall. Such heating is known in the art as “direct heating” or “direct resistance heating,” the term used herein. In this usage, “direct” denotes heating from the vessel itself, rather than through externally applied indirect resistance heating.
A major challenge in direct resistance heating is the introduction and removal of the electric current from the vessel's wall. This is not only an electrical problem, but is also a thermal problem since the conduction path can lead to unbalanced current densities that create hot spots in the conduction path. These hot spots can lead to premature material failure, such as through accelerated oxidation of the metals involved or by reaching the melting point of the metal.
One way of introducing current into a vessel's wall is through the use of an electrically-conductive metal flange. Examples of such flanges can be found in U.S. Pat. Nos. 6,076,375 and 7,013,677. The present invention is concerned with flanges used to introduce current into a platinum-containing vessel wall and, in particular, ensuring a uniform current density within the flange and the vessel varying the molten glass.